1. Field of the Invention
The present invention relates to a phase difference detection circuit for detecting a phase difference between plural signals, and an optical disk device for recording or reproducing data in an optical disk such as a CD-R/RW, a DVD-R/RW, and a DVD+R/RW, and more particularly, to a phase difference detection circuit that generates a tracking error signal, which is used as a control signal for controlling a servo motor, and an optical disk device using the phase difference detection circuit.
2. Description of the Related Art
In the related art, a so-called “three-beam method” is widely used for generating a tracking error signal in a CD (Compact Disk) reproduction device.
In a DVD (Digital Versatile Disk) reproduction device, because the track pitch of a DVD is as narrow as 0.74 μm, which is less than half of the track pitch of a CD, and the three-beam method requires much high accuracy of the device, usually, a so-called “phase difference tracking” method with one beam is used for generating a tracking error signal in the DVD reproduction device. For example, Japanese Laid Open Patent Application No. 10-275345 and Examined Japanese Patent Application (Koukoku) No. 3-18255 disclose techniques in this field.
In the phase difference tracking method, the reflected light of a main beam is received by four divisional light receiving elements A, B, C, D, and is converted into electrical signals; a signal SA and a signal SC from two diagonal light receiving elements A and C, and a signal SB and a signal SD from two diagonal light receiving elements B and D, are summed respectively, then the two summation signals S(A+C) and S(B+D) are converted into respective digital signals in a comparator. From edges of the two digital signals, a phase difference is detected by a phase comparator.
The output signal from the phase comparator is a PWM (Pulse Width Modulation) signal, which may have three levels. The PWM signal is smoothed by a low-pass filter or the like, and is output as a tracking error signal.
Compared to a push-pull method using three beams, the phase difference tracking method has an advantage in small influence due to signal levels or the track pitches.
When using the phase difference tracking method in an optical disk reproduction device, in order to reproduce lengths of pits on the recording surface of an optical disk when converting reproduction signals from the four divisional light receiving elements A, B, C and D into digital signals, analog processing, such as amplification or filtering, is performed.
FIG. 9 is a block diagram illustrating a configuration of a DPD (Differential Phase Detection) circuit, which is used as a phase difference detection circuit in an optical disk device of the related art.
As illustrated in FIG. 9, the four divisional light receiving elements A, B, C and D convert the received reflected light into reproduction signals, which are electrical signals, and the reproduction signals are amplified by corresponding amplifiers AMPa through AMPd. The signal SA and the signal SC, which are output from the two diagonal light receiving elements A and C and amplified by the amplifiers AMPa and AMPb, are summed in a calculation circuit 101; the resulting summation signal is output to a comparator 103, and is converted into a digital signal S103 in the comparator 103. The resulting digital signal S103 is input to a phase comparator 105.
Similarly, the signal SB and the signal SD, which are output from the two diagonal light receiving elements B and D and amplified by the amplifiers AMPb and AMPd, are summed in a calculation circuit 102: the resulting summation signal is output to a comparator 104, and is converted into a digital signal S104 in the comparator 104. The resulting digital signal S104 is input to the phase comparator 105.
The phase comparator 105 detects a phase difference between the digital signal S103 from the comparator 103 and the digital signal S104 from the comparator 104. When the phase of the digital signal S103 is earlier than the phase of the digital signal S104, the phase comparator 105 outputs a pulse signal (down signal DNa) at a high level having a width equaling their phase difference. Whereas, when the phase of the digital signal S104 is earlier than the phase of the digital signal S103, the phase comparator 105 outputs a pulse signal (up signal UPa) at a high level having a width equaling their phase difference.
The up signal UPa from the phase comparator 105 is smoothed by a low-pass filter 106, and is input to a non-inverted input terminal of an operational amplifier AMP, whereas the down signal DNa from the phase comparator 105 is smoothed by a low-pass filter 107, and is input to an inverted input terminal of the operational amplifier AMP.
The operational amplifier AMP differentially amplifies the input signals and outputs a tracking error signal TEA.
Specifically, when the voltage of the signal smoothed by the low-pass filter 106 is greater than the voltage of the signal smoothed by the low-pass filter 107, a tracking error signal TEA having a positive voltage is output; when the voltage of the signal smoothed by the low-pass filter 106 is less than the voltage of the signal smoothed by the low-pass filter 107, a tracking error signal TEA having a negative voltage is output.
In this way, even though the tracking error signal TEA is an analog signal, in an optical disk device for reproducing or recording data in an optical disk the tracking error signal TEA is converted into a digital signal by AD conversion in circuits at later stages of the DPD circuit 100 (not illustrated), and is used as a servo signal for controlling a servo motor (not illustrated).
However, because the phase comparator 105 and the low-pass filters 106, 107 are analog circuits, condensers and resistors are used; hence, the area of the whole circuit is large.